Polylactide or poly (Lactic acid) (PLA) is a biobased and biodegradable polymer derived from renewable resources and widely used as an alternative material to fossil-based polymers. The enhancement of the thermal stability or heat-resistant properties of PLA is a crucial issue to widen the scope of its industrial and commodity applications.
Upon mixing poly (L-lactic acid) (PLLA) with poly (D-lactic acid) (PDLA), an interesting crystalline phase will be formed. A racemic stereocomplex structure in the equimolar mixtures of PLLA and PDLA has been reported, which was defined as stereocomplex crystals. The melting temperature of the stereocomplex crystals is about 50° C. higher than that of the pure PLLA or PDLA homo-crystals. The PLA stereocomplex is expected to improve several of the properties of PLA-based materials, such as physical properties, thermal properties and hydrolysis resistance.
However, the formation of stereocomplex is affected by many factors, such as the blend ratio, molecular weight, optical purity of PLLA and PDLA, processing and heat treatment conditions. It means that there is a competitive relation between homocrystals (a crystal form) and stereocomplex crystals under certain conditions. So the PLA stereocomplex consisting solely of pure stereocomplex crystals cannot be obtained easily by blending PLLA and PDLA.
Recently, several patents have focused on how to improve the crystallization rate and how to increase the content of stereocomplex crystals. For example, CN 102532837 discloses a method of preparation after melt mixing PLLA and PDLA at a temperature of 140-210° C. Although the higher content of stereocomplex crystals is obtained by the disclosed method, the content of stereocomplex crystals decreases and the homocrystals appears in subsequent processing.
In JP 20863356 the stereocomplex crystals are obtained after annealing at a high temperature between the melting point of the homopolymer and the stereocomplex PLA. The crystallinity is very low due to the slow crystallization rate.
In JP2008248022 a diblock copolymer of PLLA and PDLA is synthesized for improving the content of stereocomplex crystals.
The method of blending nucleating agent is simple, more convenient and efficient to improve the crystallinity of stereocomplex, in comparison to heat processing at lower temperature, annealing and synthesizing. In JP2003192884 the crystallization rate is improved by adding a phosphoric ester metal salt for nucleating agent. CN 18988327 A discloses a composition comprising stereoblock PLA and an aryl urea derivative, the stereocomplex crystals are selected and the crystallinity is improved in this method. However, the content of nucleating agent is higher, and the homocrystals can also be observed in the PLLA/PDLA blends.
The inventors have now found that the stereocomplex crystals can be selected by adding a specific nucleating agent, and that it is possible to obtain a composition of pure PLA stereocomplex crystals and substantially free of PLA homocrystals.